Starting from the classical conservative form of the shallow water equations, the source term of the momentum balance equation was modified to
increase computational stability. This was achieved by replacing the term representing the local bed slope by an alternative expression of the pressure
effect due to cross-section irregularities in non-prismatic channels. The resulting mathematical model turns out to be particularly suitable for complex
channel and natural river geometries, and also improves the computational stability for complicated flow conditions. The explicit finite difference
MacCormack scheme was adopted for numerical implementation. The model was thoroughly tested using a set of numerical test cases involving
various channel geometries and a wide range of flow conditions. The same simulations were compared also with the classical model and with other
schemes. Finally, a real flood event on the Italian river Reno was simulated, to confirm the model suitability for natural channels.

Starting from the classical conservative form of the shallow water equations, the source term of the momentum balance equation was modified to
increase computational stability. This was achieved by replacing the term representing the local bed slope by an alternative expression of the pressure
effect due to cross-section irregularities in non-prismatic channels. The resulting mathematical model turns out to be particularly suitable for complex
channel and natural river geometries, and also improves the computational stability for complicated flow conditions. The explicit finite difference
MacCormack scheme was adopted for numerical implementation. The model was thoroughly tested using a set of numerical test cases involving
various channel geometries and a wide range of flow conditions. The same simulations were compared also with the classical model and with other
schemes. Finally, a real flood event on the Italian river Reno was simulated, to confirm the model suitability for natural channels.